The invention relates to a clutch system featuring an automated friction clutch with a hydraulic clutch withdrawal system, which is activated by an actuator controlled by a control unit.
Clutch systems with automated friction clutches are well known. In the case of a friction clutch activated hydrostatically by means of a hydraulic clutch withdrawal system, a master cylinder is activated by an actuator, for instance an electric motor. The master cylinder transmits the pressure generated by the master cylinder via a pressure line to the slave cylinder, which, by means of an axially displaceable piston and under interposition of a release bearing on a lever system, can constitute a diaphragm spring that pressurizes a non-rotatably and axially displaceable pressure plate connected with a clutch cover and locks against a clamping-load plate firmly connected with a clutch cover. A clutch disc with friction linings is disposed between the clamping-load plate and pressure plate, which, depending on the tension between the clamping-load and pressure plate, constitute a friction lock-up and close the friction clutch or open it upon release of the friction lock-up.
The actuator is controlled by a control unit that feeds the actuator with signal values owing to the calibration data backed-up in the control unit, for instance clutch characteristic curves; said signal values effect pre-set travel on the friction clutch. In the process, the travel in the action chain of the clutch withdrawal system, for instance, directly inside the actuator, is determined and corrected by considering the ratios between the friction clutch and sensor. In this manner, torque to be transmitted via the friction clutch can be assigned to the traveled distance.
In the case of a hydraulic clutch withdrawal system, the hydraulic section is exposed to external influences, for instance temperature or friction linings wear which lead to variation of the pressure medium volume. A connection opening—a so-called snifting groove—to a reservoir (tank) is therefore provided in the master cylinder. When an opened friction clutch is involved, opened by pressurizing the slave cylinder piston and whilst the friction clutch is closed, the connection opening can be uncovered by moving the master cylinder piston in the resting position and pressure can be equalized. When a closed friction clutch is involved, pressure can be equalized in a fully opened friction clutch, because the master cylinder piston is then in a resting position and uncovers the connection opening. In general, the connection opening is controlled by means of the master cylinder piston that uncovers the opening when passing a seal. The seal in this case is formed as a static U-seal. The master cylinder piston features axial grooves for its actuation, on its front side opposite the piston rod; these axial grooves uncover the connection opening when passing the seal. Depending on the influencing parameters, for instance the tolerance chain of the master cylinder, the state of seal wear etc., the connection opening can be uncovered at different actuation travels of the master cylinder piston, which make difficult or even impossible to obtain definite correlation between the actuation signal transmitted from the control unit to the actuator and the actually set clutch travel.